(a) Field of the Invention
The present invention concerns the heat treatments of steels before hardening, by cementation, carbonitridation and heating, in order to provide a superficial hardening of metallic pieces.
(b) Description of Prior Art
In the past, the gaseous atmospheres used during cementation, carbonitridation and heating before hardening, of steels were mostly obtained from endothermic type gas generator apparatuses.
A typical example of the composition of an atmosphere for cementation is given below:
nitrogen (N.sub.2): 40 % PA1 carbon monoxide (CO): 19 % PA1 carbon dioxide (CO.sub.2): 0.3 % PA1 hydrogen (H.sub.2): 35 % PA1 methane (CH.sub.4): 1 % PA1 water vapor (H.sub.2 O): 0.6 % PA1 oxygen (O.sub.2): traces
For carbonitridation, similar atmospheres are used, to, which ammonia (NH.sub.3) has been added which makes it possible to add nitrogen to the metal.
Presently, a proportionately high number of cementation, carbonitridation or heating plants, before hardening, of steels use industrial gases for producing their atmospheres, in preference to endothermic generators. In this case, the atmospheres resulting from the injection of a mixture of N.sub.2, CH.sub.3 OH (methanol), in some cases CH.sub.4, and NH.sub.3 in the case of carbonitridation, are prepared inside the furnaces.
Nitrogen can be obtained from:
a cryogenic plant generally located far from the user, and in this case it is delivered in gaseous form (compressed bottles) or as a liquid (storage in liquid form and vaporization before use). PA2 a non cryogenic generator located directly at the client, which is either an adsorption generator known under the name "PSA", or a generator operating by gaseous permeation, or with "membranes" for example, which is economically interesting as compared to nitrogen of cryogenic origin, but which causes problems because of the relative impurity of the gas obtained, in particular because the oxygen content is relatively high, generally of the order of 0.1 to 5 %.
If there is no additional purification, the raw nitrogen obtained is therefor impure, because it contains a small portion of oxygen and traces of water. To limit the quantity of oxygen and water, the coefficient of extraction of the generator should be lowered (flow of nitrogen obtained/flow of air treated), and its production capacity is also lowered, which is obviously detrimental to the cost of the gas treated.
By way of example, a generator of the "PSA" type usually has the following performance data as a function of the content of oxygen in the gas obtained.
______________________________________ Concentration O.sub.2 (%) 5% 1% 0.1% Production (m.sup.3 /h) 180 100 35 ______________________________________
However, for cementation and carbonitridation, a residual concentration of oxygen of the order of 2% in the nitrogen used for the N.sub.2 --CH.sub.3 OH mixtures appears suitable, since a higher concentration would cause problems in trying to obtain an atmosphere with a high carbon potential without formation of soot, while with a lower concentration, an adsorption or permeation generator would be of lesser interest on a cost point of view.
On the other hand, it should be mentioned that most of the treatments of steels, before hardening, by cementation, carbonitridation and heating are carried out in non muffle furnaces, i.e. with a simple partition of refractory bricks, without metallic partition, or muffle, so that the atmosphere inside the furnace is in direct contact with the refractory bricks which constitute the thermic insulation of the furnace. Now, refractory bricks are porous and act as sponges with respect to the atmosphere.
When such a furnace is in operation, the residual oxygen is converted into, CO, H.sub.2 O and CO.sub.2. The additional hydrocarbon makes it possible for example, to preserve a low content of H.sub.2 O and CO.sub.2 in spite of the presence of oxygen in the nitrogen, provided that the oxygen content is not too high. If this is not the case, an additional quantity of hydrocarbon which is qualified as excessive must be injected, because it can produce soot, heterogeneous cementations, and can be responsible for lowering the CO content. At the limit, it may be impossible to obtain a high potential of carbon in the atmosphere, which is obviously contrary to a good treatment.
The maximum content of oxygen in nitrogen which is compatible with most of the treatment cycles necessary during cementation, carbonitridation and heating, before hardening, of steels is of the order of 2%. In this case, the residual contents of H.sub.2 O and CO.sub.2 can be kept at low values, generally lower than 0.6% in the case of H.sub.2 O and 0.3% in the case of CO.sub.2.
However, the atmosphere which is formed inside the furnace diffuses in the refractory bricks and an equilibrium is reached at the interface bricks/atmosphere when the furnace operates continuously. However, an important problem remains during the periods when the furnace does not operate. Indeed, it happens more and more that the heat treatment plant is interrupted for relatively long periods of time, for example during the week-end. In this case, the treatment atmosphere obviously ceases to be injected in the furnace not only for economical reasons but also for safety reasons because it is potentially explosive (high content of hydrogen and CO) and toxic (high content of CO). On the other hand, the temperature of the furnace is often also somewhat lowered.
If no atmosphere is injected into the furnace, the latter tends to be filled with air which then diffuses through the refractory bricks. When the treatment is resumed, the air which is present in the furnace as well as in the refractory bricks must be flushed. This operation is long, and therefore costly, and is detrimental to the production. It is therefore usual to try to protect the furnace from air pollution during a period of non production, and for this purpose, the openings and the furnace are closed and a small flow of nitrogen, generally between 1/6 and 1/3 of the nominal flow, is injected in the furnace to maintain an over pressure preventing entry of air.
If the nitrogen used is derived from a cryogenic source, the residual content of oxygen in the furnace and in the refractory bricks remains very low., and the starting up of the furnace to resume production, called a period of reconditioning, is then very short, generally of the order of 15 minutes to a few hours depending for example on the temperature of the furnace.
If the nitrogen originates from another source and contains for example 2% oxygen, which amount is compatible with a later treatment and is particularly economical, the reconditioning of the furnace can be much longer, to the detriment of the productivity of the equipment. As a matter of fact, it is not only necessary to flush the atmosphere inside the furnace, but also the atmosphere which is present in the refractory bricks. This operation is particularly lengthy, since the bricks act as sponges and it is particularly difficult to diffuse gas therethrough. Moreover, flushing can be carried out in a known manner with the treating atmosphere which is again injected into the furnace. The latter contains a particularly high amount of hydrogen. This gas, which consists of a very "small" molecule diffuses very rapidly, so that hydrogen converts oxygen which is present in the refractory bricks into water vapor, to the extent that the water vapor content thus produced reaches 4%. This 4% content of water vapor is incompatible with the latter treatment which requires values lower than 0.6%. The water vapor must therefore be chemically destroyed or flushed. The flushing of water vapor is an operation which is always difficult since this polar molecule has the property of being very easily adsorbed at the surface of solid materials. On the other hand, refractory bricks, because of their porosity, have a very high specific surface.
The chemical destruction of water vapor is eventually carried out by reaction with a hydrocarbon such as methane, but this reaction is very slow or even nearly nonexistent the temperature is lower than 600.degree. C., which comes rapidly in the case of refractory bricks, since there is an important temperature gradient between the interior of the furnace and the outside partition thereof, whose temperature is generally lower than 100.degree. C. in a normal furnace.